Chapter 11, Pull Mail Protocols: POP and IMAP, covered servers that dealt with part of the task of mail delivery. POP, IMAP, and other pull mail protocols allow end users to retrieve their e-mail from a central mail server system. There are two other aspects of mail delivery, though: delivering the mail to the central mail server, and sending outgoing mail. As it happens, both these tasks are handled by push mail protocols, so called because the sender initiates push mail transfers. The relationship between push and pull mail protocols is covered in the "Pull Mail's Place in the Mail Delivery System" section of Chapter 11.
The most common push mail protocol in 2002 is the Simple Mail Transfer Protocol (SMTP). The vast majority of the mail delivered on the Internet passes through at least one SMTP transfer. Pull mail servers usually also run an SMTP push mail server so that they can accept mail for local delivery as well as accept outgoing mail from local systems, addressed to outside destinations. For this reason, SMTP servers are extremely important for any Linux system that's to handle mail. Every major Linux distribution ships with at least one SMTP server, but the default server varies from one distribution to another. This chapter covers the three most common Linux SMTP servers: sendmail, Exim, and Postfix. This chapter also covers a tool that's frequently used in conjunction with an SMTP server to process mail after it's been received by the server computer: Procmail. Before delving into the details of SMTP server configuration, though, it's important to understand when you should run such a server, which server is best to run, and how to configure your domain to handle a mail server.
Although many networks can make do with only minor changes to mail server configurations from the default, other networks require extremely complex mail server configurations. If you need more information than can be presented in a single chapter, you may want to obtain a book on your mail server of choice. Examples include Costales and Allman's Sendmail (O'Reilly, 1997), Hunt's Linux Sendmail Administration (Sybex, 2001), Hazel's Exim: The Mail Transfer Agent (O'Reilly, 2001), Blum's Postfix (Sams, 2001), Sill's The qmail Handbook (APress, 2001), and McCarthy's The Procmail Companion (Addison Wesley, 2001).